In today's EAT systems according to the state of the art such as, for instance, systems according to European patent specification EP-B-1 054 722, this means that the volume of the SCR catalytic converters has to be increased in order to achieve adequate nitrogen-oxide removal rates. As a rule, however, this cannot be reconciled with the installation space available.
Furthermore, a limit value for the number of particles applies to the new engine models. These limit values can no longer be met without introducing a closed wall flow filter.
German patent document DE 103 48 800 discloses a method to control a reductant feed system comprising at least one heating element. Upstream, the feed system is connected to an SCR catalytic converter. During the air-assisted injection of the reductant into the feed system, an evaporated mixture of air and reductant is formed over the heated element, and this mixture is introduced into an exhaust-gas mixture that is entering the SCR catalytic converter. An aqueous urea solution is employed as the reductant. The entire feed system, including the mixing unit for the urea-water solution and air as well as the heating element or evaporator unit, is integrated downstream from the SCR catalytic converter into the exhaust-gas carrying pipe downstream from the engine outlet. The amount of heat necessary to evaporate the mixture of urea-water solution and air is generated electrically by a 12-volt battery current.
German patent document DE 10 2007 029 674 discloses an assembly for nitrogen oxide reduction in a gas stream containing oxygen, especially in an exhaust-gas system of an internal combustion engine, comprising an SCR catalytic converter arranged in a gas line that carries nitrogen oxide and oxygen, a conversion reactor with which a solution containing an ammonia precursor is converted into an ammonia solution, and an evaporator unit configured as a separate component, which is located downstream from the conversion reactor and in which the ammonia solution is evaporated upstream from the SCR catalytic converter before being fed into the gas line. The conversion of the solution containing an ammonia precursor (preferably a urea-water solution) into an ammonia solution takes place according to the invention using microwaves. Upstream from the evaporator unit configured as a separate component, there can be a heat exchanger in order to heat up the ammonia solution so as to reduce the energy needed for the evaporation in that, for instance, the heat of the exhaust-gas return line is used. The evaporator itself is an electrically operated heating element.
German patent document DE 10 2007 042 836 discloses a device for the after-treatment of the exhaust gas of a motor vehicle using a liquid that releases ammonia, for example, a urea-water solution, comprising a reservoir to store the liquid, whereby a temperature-control device is provided for purposes of regulating the temperature of the liquid. In order to cool the liquid, the temperature-control device is associated with a cooling circuit of the motor vehicle. This can be the cooling circuit of the internal combustion engine or an air-conditioning circuit. The temperature-control device is configured as a separate component. Thanks to the temperature-control device, the liquid can be heated, thus preventing it from freezing, or else the liquid can be heated to a favorable temperature level that yields an excellent purification of the exhaust gas.
German patent document DE 10 2009 009 538 discloses a system to regulate the temperature of a fluid additive, preferably a urea-water solution, for the exhaust-gas system of an internal combustion engine, said system being characterized by means to bring about a heat exchange between the additive and a coolant circuit of a coolant installation, especially of an air-conditioning unit.
The development described in German patent document DE 10 2007 011 184 is based on the objective of further reducing the emissions of internal combustion engines, especially of diesel engines, and minimizing the installation space needed to treat the exhaust gas. For purposes of achieving this objective, a heat exchanger is disclosed for cooling the exhaust gas from an internal combustion engine of a motor vehicle in order to return the exhaust gas to the internal combustion engine. For cooling purposes, this heat exchanger has at least a first flow channel through which at least a first fluid flows, and at least a second flow channel through which a second cooling fluid flows, and this heat exchanger also has a housing to accommodate the at least one first flow channel and the at least one second flow channel. The housing has at least one inflow section through which the second fluid flows into the heat exchanger and at least one outflow section through which the second fluid flows out of the heat exchanger. The heat exchanger is characterized by at least one device that converts a liquid urea solution into at least ammonia gas.
International patent WO 2012/022687 discloses a method for operating an exhaust-gas treatment device having at least one reservoir for a reducing agent and at least one feed device for the reducing agent, whereby the method encompasses at least the following steps: a) checking the filling level of the at least one reservoir; b) checking the current exhaust-gas mass flow; c) adding reducing agent whenever the filling level of the at least one reservoir falls below a minimum filling level and the exhaust-gas mass flow is within a low-load range. In a special embodiment, one of the following actions is carried out in step c): heating at least the exhaust-gas mass flow or the reducing agent and feeding in reducing agent. In this process, the heat feed leading all the way to the reducing agent can be ensured or improved by external electric heaters.
German patent document DE 10 2009 025 135 discloses a device for evaporating a urea-water solution, comprising a conveying channel for the urea-water solution extending through at least a first zone and a second zone in order to introduce thermal energy, whereby the two zones can be heated separately from each other and, in the second zone, the conveying channel at first has a meandering course in a second inlet area, and subsequently a straight course. In the first zone, the urea-water solution is preheated to a temperature within the range from 100° C. to 180° C. [212° F. to 356° F.], while in the second zone, it is evaporated at a temperature ranging from 420° C. to 490° C. [788° F. to 914° F.].
German patent document DE 10 2008 012 087 discloses an evaporator unit for generating a gas stream containing ammonia out of a urea-water solution, said unit being likewise configured as a separate component.
Industrial engines according to the state of the art are typically operated with catalytically active exhaust-gas after-treatment (ANB) systems according to the above-mentioned European patent EP-B 1 054 722—or also called “SCR® systems” consisting of DOC/(c)DPF+SCR/ASC where a urea-water solution (HWL) is metered in upstream from the SCR/ASC catalytic converter—in order to comply with the applicable emission regulations (Tier 4 Final (US)/Stage IV (EU) and subsequent regulations).
In order for the exhaust-gas after-treatment system to be able to operate with the requisite cleaning efficiency, minimum exhaust-gas temperatures and catalyst operating temperatures of 230° C. [446° F.] or more are necessary.
In low-load operating states, the required minimum exhaust-gas temperatures can only be provided by means of engine-related heating measures (for instance, throttling the engine). The energy that is introduced into the exhaust gas in order to attain the requisite exhaust-gas temperatures is no longer available to the drive train. This translates into a diminished degree of drive efficiency for the engine and into greater fuel consumption and thus also increased CO2 emissions.
Regeneration of the closed diesel particulate filter has a very detrimental effect on fuel consumption. Exhaust-gas temperatures of about 600° C. [1112° F.] are needed so that the soot deposited in the filter can be burned off in a controlled manner. In order to achieve such temperatures, in addition to the passive regeneration, as a rule there is a need to employ active measures such as, for example, additional fuel injection and exothermal conversion of the resultant unburned hydrocarbons on the DOC or else active regeneration measures, for instance, fuel-operated burners according to European patent applications EP-A 2 177 728, EP-A 2 192 279 or international patent document WO 2010/139429.
For the operation of the SCR system, a urea-water solution containing 32.5% urea is typically used as the reducing agent. In order to release ammonia from this, 67.5% of the water first has to be evaporated and the urea has to be hydrolytically converted into ammonia and CO2.